Generally, outer body panels of a vehicle require excellent physical properties in formability, dent-resistance, corrosion-resistance and surface quality. However, the conventional AA5000-series alloy sheets have not been favoured because they have low mechanical strength even after press forming and may also exhibit poor surface quality. Therefore, the AA6000-series sheet alloys have been increasingly used. The AA6000-series alloys provide excellent bake hardenability after painting and high mechanical strength, thus making it possible to manufacture more thin-gauged and more light-weight sheets in combination with a class A surface finish.
The vehicle body parts, such as the bonnet and the like, in general, are manufactured by the mechanical assembly between inner parts and outer parts. For example, the proper length of a flange is prepared and formed at the end of an outer panel of a vehicle. An inner panel is fixed onto the inside of the outer panel, and the flange of the outer panel is bent and folded to produce a mechanical binding. The whole process described is called “hemming”.
In the hemming process, it is preferred to perform a flat hemming (180° process), which has very strict processing conditions and a relatively low ratio (r/t) between the bending centre radius (r) and the thickness of a sheet (t). However, the bending property of AA6000-series sheet products is inferior to that of AA5000-series alloys. Thus performing a flat hemming at parts where a press induced property is relatively high (i.e., a part with much transformation or deformation) results in a higher defect rate.
U.S. Pat. No. 5,266,130 discloses a process for manufacturing aluminium alloy panels for the automotive industry. The alloy includes as essential components broad ranges of Si and Mg and may also include Mn, Fe, Cu, Ti, etc.
U.S. Pat. No. 5,616,189 discloses a process for producing aluminium sheet for the automotive industry. The aluminium sheet produced from the alloys was subjected to a 5 hour pre-age treatment at 85° C. The aluminium sheet used in this patent was a continuous cast sheet. It is reported that sheet products produced by this route have been found to exhibit poor bendability.
U.S. Pat. No. 6,780,259 discloses a process for producing an aluminium alloy of the AA6xxx-series with carefully selected Mg, Si and Mn-contents. The process has a specific pre-age treatment resulting in improved bendability, including hemming, for use in forming panels for automobiles.
International application WO-98/24571 discloses a multilayer metal composite product obtained by compound strand casting. The product comprises a core, preferably of an aluminium alloy, on at least one side of the core an interlayer bonded to the core, and a cladding bonded to the interlayer. Depending on the application of the product, the cladding forming the outer surface of the composite product can be an aluminium brazing alloy for use in brazing sheet, an AA1xxx-series alloy to obtain a mirror like surface finish, or it can be a zinc containing aluminium alloy, zinc, or a zinc alloy to improve corrosion resistance.
In spite of these disclosures, there is a great need for selection of aluminium alloy sheets and methods for producing vehicle parts or members providing good strength and levels of formability which permit ease of forming into intricate parts without cracking.